The present invention relates to an image-pickup apparatus such as a video camera or a digital still camera and, more particularly, to a focus control method in the image-pickup apparatus.
A mainstream method in auto-focus (AF) control of a video camera or the like is a TV-AF method which generates an AF evaluation value signal indicating the sharpness (contrast state) of image signals generated using an image-pickup element and searches for the position of the focus lens where the AF evaluation value signal is at the maximum.
AF methods include an external ranging method (or an external phase difference detection method) in which an in-focus position of the focus lens is calculated from the distance to an object (object distance) detected by a ranging sensor provided independently of an image-pickup lens, and then the focus lens is moved to the in-focus position.
In the external phase difference detection method, a light flux received from an object is divided into two, and the two divided light fluxes are respectively received by a pair of light-receiving element arrays (line sensors). The amount of shift between images formed on the pair of line sensors, i.e., a phase difference is detected, an object distance is calculated from the phase difference by triangulation, and the focus lens is moved to an in-focus position with respect to the object distance.
There is also available an internal phase difference detection method. In the internal phase difference detection method, a light flux having passed through the exit pupil of an image-pickup lens is divided into two, and the two divided light fluxes are respectively received by a pair of line sensors. A phase difference between two images on the pair of line sensors is detected, and the amount of defocusing of an image-pickup lens is calculated from the phase difference. Then, the focus lens is moved by an amount corresponding to the amount of defocusing.
A hybrid AF method in which these AF methods are combined to make full use of high focusing accuracy of a TV-AF method and high-speed focusing of a phase difference detection method has been disclosed in Japanese Patent Laid-Open No. 2005-84426. The hybrid AF method disclosed in Japanese Patent Laid-Open No. 2005-84426 is a method in which an in-focus state is achieved by the phase difference detection method when it is determined that an in-focus state cannot be achieved by the TV-AF method because an object has low luminance or low contrast.
An image-pickup apparatus which, in the phase difference detection method, calculates the degree of coincidence between image patterns, i.e., to what degree two images on line sensors are similar to or coincide with each other, and determines the reliability of a ranging result on the basis of the degree has been disclosed in Japanese Patent No. 3548184.
An image-pickup lens for the latest video cameras can be easily reduced in size and weight and is excellent in optical characteristics. Accordingly, an image-pickup lens of the inner focus type in which a focus lens is arranged closer to an image plane than a magnification-varying lens has become the mainstream.
In an image-pickup lens of the inner focus type, the position of the focus lens where an in-focus state is achieved with respect to the same object distance varies depending on the position of the magnification-varying lens, as shown in FIG. 6. The moving amount of the focus lens with respect to the same change of the object distance (e.g., infinity to 1 m) varies depending on the position of the magnification-varying lens, as indicated by a curve D in FIG. 7. In particular, the moving amount of the focus lens tends to be larger on the telephoto side than on the wide-angle side.
However, in a conventional hybrid AF method, the focus control by the TV-AF method is first performed, and then it is determined whether or not to perform the focus control by the phase difference detection method on the basis of the state of the focus control by the TV-AF method. For this reason, the conventional hybrid AF method does not make full use of the features of the phase difference detection method, so that responsive high-speed focus control is not sufficiently realized.
If movement of the focus lens to an in-focus position, (which includes one calculated from a driving amount) caused by a change in the in-focus position calculated by the phase difference detection method is repeated, the focus lens may be moved frequently and discontinuously. This may make a captured image unstable.